Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process. This can be a reactive, benign, or pathological state, and physiologically acts to deposit connective tissue, which can obliterate the architecture and function of the underlying organ or tissue. Fibrosis can be used to describe the pathological state of excess deposition of fibrous tissue, as well as the process of connective tissue deposition in healing. While the formation of fibrous tissue is normal, and fibrous tissue is a normal constituent of organs or tissues in the body, scarring caused by a fibrotic condition may obliterate the architecture of the underlying organ or tissue.
For example, as fibrotic scar tissue replaces heart muscle damaged by hypertension, the heart becomes less elastic and thus less able to do its job. Similarly, pulmonary fibrosis causes the lungs to stiffen and impairs lung function. Fibrotic growth can proliferate and invade healthy surrounding tissue, even after the original injury heals. In most cases fibrosis is a reactive process, and several different factors can apparently modulate the pathways leading to tissue fibrosis. Such factors include the early inflammatory responses, local increase in fibroblast cell populations, modulation of the synthetic function of fibroblasts, and altered regulation of the biosynthesis and degradation of collagen. Other factors include inflammation of the nearby tissue, or a generalized inflammatory state, with increased circulating mediators.
Fibrosis includes pathological conditions characterized by abnormal and/or excessive accumulation of fibrotic material (e.g., extracellular matrix) following tissue damage. Fibroproliferative disease is responsible for morbidity and mortality associated with vascular diseases, such as cardiac disease, cerebral disease, and peripheral vascular disease, and with organ failure in a variety of chronic diseases affecting the pulmonary system, renal system, eyes, cardiac system, hepatic system, digestive system, and skin.
To date, there are no commercially available therapies that are effective in treating or preventing fibrotic disease, particularly cardiac fibrosis. Conventional treatment of most fibrosis-related disorders frequently involves corticosteroids, such as prednisone, and/or other medications that suppress the body's immune system. The goal of current treatment regimens is to decrease inflammation and subsequent scarring. Responses to currently available treatments are variable, and the toxicity and side effects associated with these treatments can be serious. Indeed, only a minority of patients respond to corticosteroids alone, and immune suppression medications are often used in combination with corticosteroids.
Right ventricular (RV) failure is the primary cause of death in pulmonary arterial hypertension (PAH), and is a source of significant morbidity and mortality in other forms of pulmonary hypertension. Production of thromboxane and F2 isprostanes, both agonists of the thromboxane/prostainoid (TP) receptor, is increased in pathological states increasing load stress, such as pulmonary arterial hypertension. The prostacyclin/thromboxane balance has been associated with cardioprotective effects under stress, probably through support of the coronary arteries. While aspirin treatment can decrease both thromboxane and prostaglandin production, it will suppress beneficial prostacyclin production and has no effect on isoprostane formation.
There are no approved therapies directed at preserving RV function. F-series and E-series isoprostanes are increased in heart failure and PAH, correlate to the severity of disease, and can signal through the thromboxane/prostanoid (TP) receptor, with effects from vasoconstriction to fibrosis. Loss of RV function can progress despite treatments decreasing pulmonary arterial pressure. RV response to chronic pressure overload can take both adaptive and maladaptive forms, which often determines clinical outcome. Adaptive ventricular hypertrophy with increased protein synthesis sustains function, while fibrosis and cardiomyocyte hypertrophy can cause arrhythmias and contractile dysfunction, and maladaptive dilatation is associated with RV failure. Consequently, treatment strategies promoting adaptive hypertrophy in the face of chronic load stress could preserve cardiac function and improve outcomes.
The development of cellular hypertrophy and myocardial fibrosis that occurs with chronic pressure overload is also associated with increased oxidative stress and lipid peroxidation. The 15-F2t isoprostane (8-isoPGF2α, or 8-isoF) is a common biomarker for oxidative stress, and its levels increase with ventricular dilatation and correlate with the severity of heart failure. In addition to being a biomarker, it is suggested that 8-isoF and other isoprostanes can play a direct role in cardiomyopathy. F-series and E-series isoprostanes are known to signal through the thromboxane/prostanoid (TP) receptor, with effects ranging from vasoconstriction to fibrosis. The cyclooxygenase (COX) products of cyclic endoperoxide (PGH2) and thromboxane A2 (TxA2) are also ligands of the TP receptor, and TP receptor activation contributes to cardiac hypertrophy in models of chronic hypertension and decreases cardiac function in Gh-overexpressing mice. The TP receptor is found not only in platelets and vessels but also the right ventricle, where receptor density is increased in PAH patients.